1. Field of the Invention
This invention relates to optical fibres and optical fibre devices such as, for example, optical fibre Bragg gratings and single frequency optical fibre lasers.
2. Description of the Prior Art
Optical fibre Bragg gratings are periodic refractive index modulations impressed in either the cladding or the core (or both) of an optical fibre. In order to write the grating, a suitable fibre usually has a photosensitive core and/or a photosensitive cladding. A glass is photosensitive when its refractive index can be modified (usually, for these purposes, substantially permanently) by optical radiation.
A uniform fibre grating reflects light only at a certain resonant wavelength (the Bragg wavelength) characteristic of the grating pitch, fibre parameters and the transverse field distribution of the guided light. As a narrow band device, it has many applications such as reflectors for fibre lasers, band-stop filters, band-pass filters, or sensing elements in optical fibre sensors. An important application of fibre gratings is in single frequency fibre lasers.
A single frequency fibre laser can be constructed in two different ways. In the first implementation, a distributed Bragg reflector (DBR) laser, two reflectors are placed a short distance apart in a doped fibre (typically a few centimeters apart). One of the reflectors is a fibre grating which only reflects a narrow wavelength band of light around the Bragg wavelength; the other one can be a fibre grating or a broad band reflector such as a metal coated mirror. When the cavity length is in the other of few centimetres, the longitudinal modes of the cavity are spaced far apart. If only one of these longitudinal modes lies within the reflection spectrum of the narrow band reflector, the laser operates at single longitudinal mode. The laser can be tuned either by heating, stretching or compressing the fibre grating or gratings to change their reflection response publication reference 1!.
In a second implementation, a distributed feedback (DFB) laser, a single grating is written in which the refractive index modulation has an abrupt .pi./2 phase shift at a point along the grating's length. This implementation potentially offers more stable mode operation.
A very common fibre for implementation of single frequency fibre laser is erbium-doped germanosilicate fibre, typically co-doped with aluminium. The erbium doping gives a lasing wavelength around the telecommunication system operation wavelength of 1.55 .mu.m (micrometers), and the germanium content gives the fibre photosensitivity which allows gratings to be written in this fibre easily. The laser is typically pumped by a readily available 980 nm (nanometer) semiconductor laser diode. However, this arrangement has the disadvantage that even with the highest possible erbium doping level available in this type of glass fibre, only a small proportion of the available pump light can be absorbed. This gives a low efficiency and only a small output power from the laser, typically less than 1 mW (milliWatt). This is not sufficient for most applications. A master oscillator post-amplifier (MOPA) arrangement can be used to increase the output power, in which the unabsorbed pump (emerging from the laser with the laser output) is used to pump a section of the fibre downstream of the laser, to act as an amplifier to give a few mW of total output power 2,3!. However, the low output power of the master oscillator laser means that the noise of a MOPA device is usually high.
An alternative is to use an erbium and ytterbium doped fibre. The ytterbium ions can be pumped at approximately 980 nm, but with some two orders of magnitude larger absorption than that of an erbium (only) doped system. The pump energy absorbed by the ytterbium ions eventually transfers to erbium ions which in turn lase at approximately 1.55 .mu.m. This provides a very efficient single frequency laser of few centimeters long with output power in the range of tens of mW. An efficient energy transfer from ytterbium to erbium requires a high phonon energy glass host. The best efficiency demonstrated so far is achieved in phosphosilicate fibre doped with some aluminium.
However, the erbium/ytterbium fibre has the disadvantage that useable gratings can only be written in such fibres with hydrogenation.sup.1 which, although efficient, makes the writing more difficult and reduces the laser efficiency by introducing a background pump absorption 4!. FNT .sup.1 A low temperature hydrogen loading technique allowing very strong gratings to be written. A pre-fabricated optical fibre is placed in a high pressure hydrogen cell at room temperature for a few days to a few weeks to allow hydrogen to diffuse into the core region of the fibre. A grating is written in the fibre before the hydrogen diffuses out.
Co-doping of erbium/ytterbium doped fibres with tin has also been attempted, which does allow gratings to be written 5!, but the photosensitivity is still not strong enough to allow gratings to be written with ease, nor to allow very strong gratings to be written for laser implementations. Another disadvantage of using tin doping is that it affects the optimal glass composition for efficient energy transfer from ytterbium to erbium ions. A reduction of laser efficiency has been seen 5!.
A further problem with lasers based on previously proposed fibre gratings is that the output at the two different polarisation modes from a laser are at different wavelengths due to the birefringence of the cavity (so that the different polarisation sees a different effective refractive index, and therefore a different optical cavity length). This is normally not desirable, as it introduces an extra wavelength component in the laser's output spectrum, broadening the spectrum.
It is an object of the invention to provide a fibre which allows a strong grating to be written to provide an efficient laser, but without the difficult fabrication steps of hydrogen loading or the detrimental effects of certain other co-dopants.